With the progress of mobile electric or electronic devices such as mobile computers and mobile phones, the general trends in users' needs in recent years are directed toward smaller and lighter products and it is highly required that these products have mechanical properties such as thin thickness, high strength and high stiffness. For example, the housings of electric or electronic devices are required to be light in weight, and when an external load acts on the housing, it is necessary to avoid that the housing is at least partially bent to contact or break any inside part, or that the housing per se is broken. However, members having high stiffness and high strength such as metallic members have a problem that the weight of the product is likely to be high, since they have high specific gravities. To avoid this problem, resin members light in weight and excellent in mass productivity are increasingly used as materials forming the housings.
In the case where resin members are used to produce a housing, a method of producing a housing comprising members designed to bear respectively different functions of the housing by integrally injection-molding, for example, multiple resin members having different functions and properties such as stiffness and light weight are employed in order to respond to diverse needs. Patent Literature 1 discloses a technique of producing an electronic device housing excellent in joint strength and mass productivity by injection-molding a thermoplastic resin member containing glass fibers to a thermosetting resin member containing carbon fibers via an adhesive layer formed of a nonwoven fabric of a thermoplastic resin, in order to bond and integrate both the members.
However, this technique requires an adhesive layer formed of a thermoplastic resin at the bonding interface between the two resin members. Consequently the material cost becomes high, and there is an extra thickness of the adhesive layer formed of a thermoplastic resin in addition to the basic thickness of the product. Further, the two resin members must be overlaid on each other in the thickness direction. Accordingly, this technique has a problem of being unsuitable for further thinning the product. Moreover, since this technique is intended to bond a thermosetting resin member and a thermoplastic resin member to each other, it is not suitable for bonding two thermoplastic resin members to each other.
As an example of bonding two thermoplastic resin members to each other, Patent Literature 2 discloses a technique in which a highly stiff and hard thermoplastic resin plate and a highly flexible and soft thermoplastic resin are insert-molded and integrated in a mold, to produce a molded composite resin article suitable as an interior part of a motor vehicle, etc. This technique does not require the extract thickness for bonding unlike the technique disclosed in Patent Literature 1, and a molded article having a basic product thickness maintained and having few defects (drops) due to the resin shortage on the surface can be obtained.
However, this technique has a problem that burrs are likely to be formed at the tip of a tapered portion formed at an edge of the hard resin plate. If burrs are formed in a molded article, the step of removing the burrs must be added disadvantageously in the light of mass productivity.
Further, in the injection molded article produced by this technique, one of the two resin members is supported by the other resin member on the entire surface from below. The other resin member does not have the thickness required in the molded article. For this reason, there is a problem that the molded article is likely to be warped due to the difference between the resins in molding shrinkage.
As can be seen from the prior art as described above, an injection molded article having two resin members bonded and integrated by injection molding, which is thin without the extra thickness for bonding, has a high strength and high mass productivity and suffers little resin shortage (drops) and little resin surplus (burrs) on the surface, is not known hitherto. Further, the method for producing such an injection molded article is not known hitherto either.